Autism spectrum disorders include a group of serious and enigmatic neurobehavioral disorders that usually become apparent early in childhood and persist as lifelong disabilities. Disturbances in three categories of behavior (reciprocal social interactions, verbal and nonverbal communications, and age appropriate activities and interests) are considered hallmarks of autism.
The number of children diagnosed with autism has greatly increased in recent decades. At the midpoint of the 20th Century, autism was narrowly defined and uncommonly diagnosed (with a prevalence of about four per 10,000). Greater awareness, availability of services, changes in diagnostic criteria to include a broader spectrum of neurodevelopmental abnormalities, and possibly other factors have contributed to the ten-fold or greater increase in the frequency with which autism spectrum disorder is diagnosed (current prevalence of 40-60 per 10,000). One extraordinary aspect of the epidemiology is the three-fold to six-fold excess of males.
Autism appears causally heterogeneous. Although scientists have long abandoned the idea that autism is caused by humorless and rigid parenting, they have been unable to identify specific cause(s) in any substantial proportion of cases. Standardized criteria for autism (DSM IV-TR) may be assessed based on parental, caregiver, and/or examiner observations using the Autism Diagnostic Interview, Revised (ADI-R) and Autism Diagnostic Observation Schedule (ADOS). A small percentage of patients will have coexisting genetic disorders and an even smaller percentage a history of an environmental insult.
Only meager evidence exists to suggest that environmental insults play a significant role in the causation of autism. Prenatal and postnatal infections (rubella, cytomegalovirus, herpes) have been documented in a few cases. Little evidence exists to suggest injury in the perinatal period as a causative factor, although low birth weight and premature birth has been noted as a risk factor (Schendel et al. 2008). Although autism has been reported among infants with prenatal exposure to thalidomide, cocaine, alcohol, and valproate, most infants prenatally exposed to these and other drugs or chemical agents do not develop autism. Considerable attention has been given to the concept that immunizations for measles, mumps, and rubella (MMR) might cause autism. However, repeated study has not provided evidence to support this theory.
No laboratory finding is consistently abnormal, although plasma serotonin levels may be elevated in affected individuals and first-degree relatives. In a promising study, Nelson et al. (2001) found several neuropeptides and neurotrophins (vasoactive intestinal peptide, calcitonin gene-related peptide, brain-derived neurotrophic factor and neurotrophin 4/5) to be elevated in newborn blood spots from infants who were later found to have autism. Confirmation of these findings has not been reported by other investigators. More recently, James et al. (2006, 2008) have proposed that metabolic vulnerability to oxidative stress may be an autism susceptibility factor, and Carter (2007) has suggested that the skewed male:female ratio in autism may be explained by sex-specific responses to the neuropeptides, oxytocin and vasopressin.
The genetic contribution to the causation/predisposition to autism is considered to be substantial on the basis of high concordance in monozygous twins, a recurrence rate of about 5% among siblings, the uniquely high male:female ratio (about 4:1 in most studies), the co-occurrence of autism with a number of single gene disorders and chromosome aberrations, and the presence of behavioral disturbances among first degree relatives. These considerations aside, no specific genetic cause has been found to explain more than 1-2% of autism cases, and overall only in 10-20% of autism cases can a cause be determined.
The strongest evidence for a heritable basis of autism comes from twin studies. Overall, these studies show high concordance of autism among monozygous (MZ) twins and low concordance among same sex dizygous (DZ) twins, resulting in greater than ninety percent heritability estimates. Four prominent studies dealing specifically with autistic disorder (narrowly defined to exclude Asperger disorder and pervasive developmental disorder) report concordance of 36-96% in MZ twins and 0-30% in same-sex DZ twins (Folstein and Rutter 1977, Ritvo et al. 1985, Steffenburg et al. 1989, Bailey et al. 1995).
Chromosomal abnormalities have been found in a number of individuals with autism. These include marker chromosomes, microdeletions and microduplications, rearrangements, and autosomal fragile sites (Schroer et al. 1998, Ullmann et al. 2007, Morrow et al. 2008, Freitag 2007, Sebat et al. 2007, Weiss et al. 2008, Marshall et al. 2008). Taken together, these observations do not suggest a single underlying chromosomal aberration, but rather that a variety of chromosomal changes may disturb brain development and function in a way that leads to autism. Chromosome aberrations observed in more than one case of autism include: 1q deletion, 15q deletion or duplication, 16p deletion or duplication, 17p deletion, 18q deletion, 22q deletion, and Xq28 deletion or duplication.
Several single gene entities have been found in association with autism. Most notably are the fragile X syndrome, Rett syndrome, tuberous sclerosis, phenylketonuria, Angelman syndrome, and adenylosuccinate lyase deficiency. Mutations in the neuroligins, neurexins, GABA receptors, reelin, ENGRAILED 2, SLC6A4 serotonin transporter, glutamate receptor 6, DHCR7, DLX5, MET, RPL10, SHANK3, CNTNAP2, BDNF, and other genes have been associated with autism and are properly considered candidate autism susceptibility genes (Schanen 2006, Freitag 2007, Abrahams and Geschwind 2008).
A number of genome-wide screens to identify chromosomal regions linked to autism susceptibility have been reported. The linkage evidence appears greatest for one or two loci on chromosome 7q, and loci on chromosomes 1q, 2q, 3p, 3q, 5p, 6q, 9q, 11p, 15q, 16p, 17q, and 19p. The study of candidate genes within the linkage regions has failed to identify genes that clearly cause or strongly predispose to autism.
Although several X-linked genes (NLGN3, NLGN4, RPL10, FMR1, MECP2 and ten others noted with an asterisk in the Table and FIG. 1) have been associated with autism or autistic features in males, X-chromosomal loci have not been implicated in autism by linkage analyses. This may be explained in part by existence of multiple X loci of importance (heterogeneity) or by the uninformative nature of the sib-pairs used in the analysis. Of greater importance is that linkage analysis would not detect epigenetic modifications of gene(s) on the X chromosome.
The recurrence rate in brothers and sisters of affected persons is 3-8%. This recurrence rate is less than expected if all cases were caused by autosomal recessive gene mutations (25%) or autosomal dominant gene mutations (50%). The rate is not unlike that found in conditions considered to have multifactorial causation, such as neural tube defects and cleft lip/palate. Multifactorial causation implies a collaboration between multiple genetic factors and environmental influences.
Soon after the discovery of the correct number of chromosomes in humans, the importance of gene dosage to human development and health was appreciated. Inactivation of an X chromosome in normal females, as reflected in formation of the sex chromatin body, was recognized and considered to equalize (at 1N) the dosage of X-linked genes between females and males (Lyon 1961). Trisomies, which augmented the gene dosage (to 3N) for genes on individual chromosomes, were found to be uniformly associated with mental defect and malformation, in most cases lethal before birth. Deletions of small segments of the genome, which reduced gene dosage segmentally to 1N, were likewise found to be associated with malformations and mental retardation—e.g., Cri du Chat, Miller-Dieker, Smith Magenis, velocardiofacial, Wolff-Hirschhorn, and other microdeletion syndromes (Schinzel 1988, Pai et al. 2002). These findings and others reinforce the concept that diallelic expression of autosomal genes and monoallelic expression for genes on the sex chromosomes are the norm for humans.
In the 1980s and 1990s, evidence was found, initially in mice and then in humans, that diallelic expression was not the norm for all autosomal gene loci (Nicholls et al. 1989, Engel and DeLozier-Blanchet 1991). Rather, expression of certain genes, found in specific clusters on several autosomes, was noted to be monoallelic, while the second allele, although present, was silenced. Further, the expressed gene was consistently derived from the same parent and the silenced gene from the other. Such parent specific influence on gene activity was designated imprinting.
Appreciation for the role of imprinted genes in the causation of human disease has grown steadily since these initial findings. Currently, at least twelve human chromosomes (1, 4, 6, 7, 8, 10, 11, 14, 15, 18, 19, and 20) are known to harbor gene loci that are imprinted. Genome-wide, however, less than 1% of autosomal genes appear to be imprinted, and hence have monoallelic and parent specific expression.
Several lines of evidence link imprinted regions and autism predisposition. Schanen (2006) has pointed out that the gene loci with suggestive or possible linkage to autism overlap or are in close proximity to regions subject to genomic imprinting. The evidence is strongest for loci on 7q and 15q. Duplication of proximal 15q, when derived from the mother, has been associated with autism. When duplication from the same region is derived from the father, autism does not occur. Deletions of the maternal copy of the same region on 15q, paternal disomy, and mutations of UBE3A cause Angelman syndrome, a disorder that typically presents with autistic features and mental retardation. Hence, silencing or overexpression of genes from this region appears to predispose to autism. Jiang et al. (2004) have proposed a mixed epigenetic and genetic model for autism based on abnormal DNA methylation at the 5′ CpG island of UBE3A, on chromosome 15, in one autism brain, decreased E6-AP protein (product of UBE3A) in several autism brains, and sharing of paternal 15q alleles in one cohort of autism sibpairs. MECP2, the X chromosomal gene responsible for Rett syndrome, exerts its effect by binding individual CpGs and recruiting other repression-related factors. A primary target of this effect is UBE3A, the gene responsible for Angelman syndrome. This may explain the clinical similarities between Rett and Angelman syndrome. Hussman (2001) and Martin et al. (2000) have suggested that GABAergic inhibition predisposes to autism. Three GABA receptor subunits (GABA α, β, γ) are located near the imprinted region of chromosome 15q.
Less work has been performed on the putative autism loci on 7q. Campbell et al. (2006) have found the C allele in the promotor of MET receptor tyrosine kinase in the 7q31 autism candidate region to be a risk factor in autism. The C allele reduces MET expression and alters binding of transcription factors. Schanen (2006) reviewed the status of genes in the imprinted cluster on 7q21.3 finding several attractive candidates, but none that had conclusive evidence for autism susceptibility. Freitag (2007) suggested that RELN, LAMB1, and EN2 were perhaps the three most promising autism candidate genes on chromosome 7.
Skuse (2000) has proposed that at least one loci on the X chromosome is imprinted, being expressed only from the paternal chromosome. Patients with Turner syndrome (45,X) who have a maternal X are more vulnerable to impairments in language and social interactions, whereas those with a paternal X may be protected, having significantly better social adjustment and superior language skills. Xp deletions of the boundary between the Xp pseudoautosomal region and marker DXS7103 have been found in three females with autism (Thomas et al. 1999).
Silencing and overexpression of genes with normal DNA sequence on the single X chromosome in males and the active X chromosome in females have been identified as biological phenomena of significance. The most prominent example of gene silencing occurs in the fragile X syndrome. The pathology is based on expansion and methylation of a CGG repeat in the 5′ untranslated region of the FMR1 gene. Methylation and silencing of expression typically occur when the CGG repeat number exceeds 200 copies (Lubs 1969, Sutherland 1977, Oberle et al. 1991, Yu et al. 1991). Overexpression of MECP2 caused by duplication of the gene and adjacent region in Xq28 has been documented in numerous cases of males with mental retardation, hypotonia, and recurrent infections (Pai et al. 1997, Lubs et al. 1999, Van Esch et al. 2005, Friez et al. 2006). Males with duplication of MECP2 likewise have an increased risk of autism spectrum disorders (Meins et al. 2005, Friez et al. 2006).
The various chromosomal alterations and gene mutations currently reported in association with autism indicate the genetically heterogeneous nature of autism and taken together account for only a minority (less than 20%) of cases.